Shaped spin valve type magnetoresistive transducer and method for fabricating the same incorporating domain stabilization technique

ABSTRACT

A magnetoresistive transducer and method for manufacturing the same includes a spin valve structure comprising a pinned, bottom ferromagnetic layer and an active, top ferromagnetic layer separated by a thin nonmagnetic metal spacer layer. The active ferromagnetic layer and underlying spacer layer are formed into a mesa structure having tapered opposing sides to promote better surface planarization in a thin film fabrication process. A pair of permanent magnet layer portions may be deposited at the end portions of the spin valve structure in a generally coplanar relationship to promote domain stabilization but may also be separated therefrom by a relatively thin separation layer. The magnetic read track width of the device can be accurately and reproducibly determined by photolithographically defining the spacing between the permanent magnet layer portions overlying the active ferromagnetic layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of copending U.S.patent application Ser. No. 08/401,553, filed Mar. 9, 1995. The presentinvention is related to the subject matter disclosed in U.S. patentapplication Ser. No. 07/975,479 to J. L. Nix et al. for"Magnetoresistive Device and Method Having Improved Barkhausen NoiseSuppression" filed Nov. 12, 1992, assigned to Rocky Mountain Magnetics,Inc., assignee of the present invention, the disclosure of which isspecifically incorporated herein by this reference.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to the field ofmagnetoresistive ("MR") spin valve ("SV") devices and methods forfabricating the same. More particularly, the present invention relatesto a shaped magnetoresistive spin valve device design and process formanufacturing the same for use as a magnetic transducer or "head" forreading data signals encoded on a magnetic mass storage medium.

Magnetoresistive devices or heads exhibiting so called giantmagnetoresistance ("GMR") are of current technological interest in anattempt to achieve high areal density recording in magnetic computermass storage disk drives and tapes. The GMR effect was first describedby M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff,P. Etienne, G. Creuzet, A. Friederich and J. Chazelas in Phys. Rev.Lett. 61, 2472 (1988). Typically, the magnitude of the magnetoresistiveratio ("ΔR/R") for GMR materials exceeds that of anisotropicmagnetoresistive ("AMR") materials which are currently in use asmagnetic read-transducers.

The spin valve effect is one known way to utilize GMR as described by B.Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit andD. Mauri, Phys. Rev. B 43, 1297 (1991). A typical spin valve MR devicecomprises two thin ferromagnetic layers separated by a nonmagnetic metalspacer. The magnetization of one ferromagnetic layer is allowed to movefreely, whereas the other one is pinned by an adjacent antiferromagneticor permanent magnetic layer. Essential to the operation of any type ofGMR structure is the fact that the MR response is a function of theangle between two magnetization vectors corresponding to the sensingfield.

A number of patents have previously described various deviceimplementations utilizing the spin valve effect. See for example U.S.Pat. No. 5,159,513 to Dieny et al. for "Magnetoresistive Sensor Based onthe Spin Valve Effect" issued Oct. 27, 1992; U.S. Pat. No. 5,206,590 toDieny et al. for "Magnetoresistive Sensor Based on the Spin ValveEffect" issued Apr. 27, 1993; U.S. Pat. No. 5,287,238 to Baumgart et al.for "Dual Spin Valve Magnetoresistive Sensor" issued Feb. 15, 1994; andU.S. Pat. No. 5,301,079 to Cain et al. for "Current BiasedMagnetoresistive Spin Valve Sensor" issued Apr. 5, 1994, all assigned toInternational Business Machines Corporation.

The stacked, orthogonal structures of the various device implementationstherein described locate a lower ferromagnetic layer (on which thefreely rotating magnetization vector resides) above the substrate butbelow the upper ferromagnetic layer having its magnetization vectorpinned by an adjacent antiferromagnetic pinning layer. Although thefirst listed patent indicates that the structure might be inverted,wherein the latter pinned ferromagnetic layer underlies the formerfreely rotating ferromagnetic layer, it nevertheless appears that itwould be difficult to achieve a sufficiently high current density withthe design described to provide an enhanced sensor output signalinasmuch as high resistivity materials (such as the capping layer forexample) are interposed between the current leads and the top-mostferromagnetic layer. Moreover, whether inverted or not, the read trackwidth cannot be accurately or reproducibly defined without difficultydue to the fact that both the current leads and the contiguous permanentmagnet regions may effect it and precise track width definition throughphotolithographic processing of the relatively thick conductive layer isproblematic at best. Further, precise control of the domainstabilization layer is also extremely difficult due to the relativethinness of the layers involved (on the order of less than 100 Å) andthe fact that the total magnetic moment ("M_(r) ·t") which determinesthe strength of the stabilization is limited by the thickness of thecontiguous permanent magnet layer. Utilizing a thicker permanent magnetlayer to compensate for these shortcomings could have the unintendedconsequence of altering the magnetization of the pinned ferromagneticlayer.

SUMMARY OF THE PRESENT INVENTION

A benefit of the shaped spin valve type magnetoresistive transducer andmethod for fabricating the same of the present invention is to improvethe magnetic performance of conventional spin valve devices by shapingthe head structure and adding a domain stabilization scheme to suppressthe Barkhausen noise which results from unstable magneticcharacteristics, such as multidomain formation, within the spin valvedevice.

A spin valve device in accordance with the present invention includes amagnetoresistive spin valve structure comprising two thin ferromagneticlayers separated by a thin nonmagnetic metal spacer. A pair of permanentmagnet layers is deposited at the end portions of the spin valvestructure in a generally coplanar relationship but separated therefromby a relatively thin separation layer. A further embodiment of a spinvalve device in accordance with the present invention includes atapering of the side portions of the active sensor layer (on which themagnetization can rotate freely) and the underlying spacer layer topromote better surface planarization in a thin film fabrication process.In accordance with the structure and method of the present invention,higher current density to the active sensor layer can be delivered froma pair of current leads due to the provision of relatively short andless resistive current paths. In a more specific fabrication process inaccordance with the present invention, the magnetic read track width ofthe device can be accurately and reproducibly determined byphotolithographically defining the spacing between the permanent magnetlayer portions overlying the top ferromagnetic layer.

Broadly, what has been disclosed is a spin valve type magnetoresistivesensor incorporating active and pinned ferromagnetic layers separated bya non-magnetic spacer layer formed on a substrate having an overlyingunderlayer formed thereon. The improvement comprises a pinning layerformed on the underlayer, with the pinning layer underlying the pinnedferromagnetic layer. The spacer and active ferromagnetic layersoverlying the pinned ferromagnetic layer are formed with tapered sideportions defining a mesa-like structure. In a more specific embodiment,relatively thin separation layer portions overlie at least the taperedside portions of the mesa together with corresponding permanent magnetlayer portions defining the read track width. Current may be applied tothe sensor by means of a pair of current leads contacting the permanentmagnet layer portions.

Also disclosed is a magnetoresistive device comprising a substrate, anunderlayer overlying the substrate and a pinning layer overlying theunderlayer. A first ferromagnetic layer overlies the pinning layer and aspacer layer overlies a first selected portion of the firstferromagnetic layer. The spacer layer presents a first mesa structurehaving opposing first and second sides thereof. A second ferromagneticlayer overlies the first mesa structure of the spacer layer, the secondferromagnetic layer also presenting a second mesa structure havingopposing first and second sides thereof. First and second separationlayers overly the first ferromagnetic layer contiguous with the firstand second mesa structures at the first and second sides thereof. Firstand second permanent magnet layers overlie the first and secondseparation layers and a preselected portion of an upper surface of thesecond mesa structure adjoining the opposing first and second sidesthereof. First and second conductor layers overlie a selected portion ofthe first and second permanent magnet layers respectively and a cappinglayer overlies the first and second conductor layers, the first andsecond permanent magnet layers and the upper surface of the second mesastructure not overlain by the permanent magnet layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and objects of the present inventionwill become more apparent and the invention itself will be bestunderstood by reference to the following description of a preferredembodiment taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional, air bearing surface ("ABS") viewof a prior art spin valve device substantially as described in theafore-described U.S. Patents wherein the read track width "TW₁ " isdefined by the metallization of the current leads overlying the pinned,upper ferromagnetic layer;

FIG. 2 is a schematic cross-sectional ABS view of a shaped spin valvedevice in accordance with the present invention wherein the read trackwidth "TW₂ " is defined by the permanent magnet layer portions overlyingthe top ferromagnetic layer;

FIG. 3 is an initial cross-sectional ABS view of a possible devicefabrication flow for manufacture of a shaped spin valve device as shown,for example, in FIG. 2 illustrating the substrate and overlyingunderlayer;

FIGS. 4A-4C are follow-on cross sectional ABS views of the substrate andunderlayer depicted in FIG. 3 illustrating alternative fabrication ofbuffer and pinning layers underlying the bottom ferromagnetic layer inthe manufacture of a shaped spin valve device in accordance with thepresent invention;

FIG. 5 is an additional follow-on cross sectional ABS view of thestructure of FIGS. 4A-4C further illustrating a spacer layer interposedbetween a top ferromagnetic layer and the pinned, bottom ferromagneticlayer in a self-aligned photolithography process in accordance with thepresent invention wherein photoresist is patterned on the topferromagnetic layer to produce a shaped spin valve device;

FIG. 6 is an additional follow-on cross sectional ABS view of thestructure depicted in FIG. 5 illustrating the formation of a mesa havingsloped side portions formed by ion milling of the top ferromagnetic andspacer layers in the area surrounding the patterned photoresist;

FIG. 7 is an additional follow-on cross sectional ABS view of thestructure depicted in FIG. 6 illustrating the addition of relativelythin separation layer portions overlying the sloped side portions of thetop ferromagnetic and spacer layer mesa also having overlying permanentmagnet layer portions for defining the read track width in addition to apair of current leads contiguous therewith; and

FIG. 8 is a final follow-on cross sectional ABS view of the structuredepicted in FIG. 7 illustrating the addition of a capping and gap layeroverlying the shaped spin valve device structure.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference now to FIG. 1, a prior art spin valve sensor 10 structureis depicted as described and illustrated in the aforementioned U.S.Patents. The prior art spin valve sensor 10 comprises, in pertinentpart, a substrate 12 having an overlying underlayer 14. The active spinvalve structure itself comprises two ferromagnetic layers (formed oftransition metals or alloys such as permalloy) illustrated as a lowerferromagnetic layer 16 and upper ferromagnetic layer 24 separated by anonmagnetic spacer layer 18 (formed of a noble metal such as Cu, Ag, orAu). A pinning layer 26, which may comprise an antiferromagnet such asFeMn, is deposited on top of the upper ferromagnetic layer 24 to offeran anisotropic exchange coupling of on the order of several hundredOersteds ("Oe"). Therefore, the direction of the magnetization of theupper ferromagnetic layer 24 is pinned with its easy axis perpendicularto the air bearing surface (ABS). Alternatively, the pinning layer 26can be formed at the bottom of the spin valve structure as suggested byU.S. Pat. No. 5,159,513. The underlayer 14 (such as Ta) and a cappinglayer 28 (which may also comprise Ta) are provided to protect the activestructure. Permanent magnet layers 20, 22 are formed at the sides of thesubstantially orthogonal, stacked structure to offer domain control onthe lower ferromagnetic layer 16.

The prior art spin valve sensor 10 design presents potentialdisadvantages in terms of manufacturing reproducible magnetic read headproducts inasmuch as a considerable amount of current density isrequired to deliver current to the spin valve structure through thenonmagnetic pinning and capping layers 26, 28, all of which haveinherently high resistivities. Further, the read track width ("TW₁ ")may not be precisely determined because the conductor (or current lead30, 32) layers are relatively thick compared to the active magneticsensor structures and both the current leads 30, 32 and the permanentmagnet layers 20, 22 could define the track width. This is undesirablein terms of stable read head operation. Moreover, precise control of thethickness of the pair of permanent magnet layers 20, 22 for Barkhausennoise suppression purposes is very difficult due to their limitedthickness scale which is only on the order of several tens of Angstroms("Å"). For example, when a thicker permanent magnet layer 20, 22 isneeded to adjust the total magnetic moment (the remnant magnetizationtimes film thickness, M_(r) ·t) which, in turn, determines the strengthof the stabilization, the permanent magnet layer 20, 22 thickness maybecome greater than the combined thickness of the spacer and upperferromagnetic layers 18, 24. This is inherently unsatisfactory becausethe magnetization of the otherwise firmly pinned upper ferromagneticlayer 24 could be altered.

With reference to FIG. 2, a representative example of a shaped spinvalve sensor 50 in accordance with the present invention is shown. Inthe design of the shaped spin valve sensor 50, a substrate 52 has anoverlying underlayer 54 formed thereon and a high resistivity pinninglayer 56 is deposited prior to any of the ferromagnetic layers.Deposition of the pinning layer 56 is followed by the deposition of athin, bottom ferromagnetic layer 58. A spacer layer 60 and a second, topferromagnetic layer 62 are deposited next and patterned into a mesa-likestructure with tapered sides. A shallow taper angle (on the order ofsubstantially 45° or less) has been found to be advantageous in order toachieve overall smoother device topology in fabrication. The taperedside mesa structure offers relatively large area surface planarizationwhich can be beneficial for device construction for use as magneticwrite transducers. Thin non-magnetic separation layers 64, 66 (such asCr) are deposited prior to the permanent magnet layers 68, 70 on thebottom ferromagnetic layer 58 and the sides of the mesa (as well as aportion of the upper surface of the mesa adjoining the tapered sides) toprevent exchange coupling which could result in rotating the pinnedmagnetization direction of the bottom ferromagnetic layer 58. Theformation of current leads 72, 74 follow to make contact directly withthe permanent magnet layers 68, 70. A capping layer 76 overlies thecurrent leads 72, 74, a portion of the permanent magnet layers 68, 70and the uncovered portion of the top ferromagnetic layer 62 forming thetop of the mesa structure.

In the shaped spin valve sensor 50 of the present invention, it is thepermanent magnet layers 68, 70 that define the read track width ("TW₂ ")and not the current leads 72, 74. In this manner, reproducible trackwidth control is made possible because the permanent magnet layers 68,70 are typically thinner than the conductor layer forming the currentleads 72, 74 thereby obviating the latters' more difficultphotolithographical challenges. Moreover, because current can then bedelivered directly to the active, top ferromagnetic layer 62 through thepermanent magnet layers 68, 70 and thin separation layers 64, 66, highercurrent density can be achieved than in prior art designs.

Referring additionally now to FIGS. 3-8, a representative process flowfor manufacturing a shaped spin valve sensor 50 in accordance with thepresent invention is shown. With initial reference to FIG. 3, asubstrate 80 is illustrated having an overlying underlayer 82 as shownfor subsequent deposition and patterning of the spin valve structure.The spin valve structure, more fully described hereinafter, can also bedirectly prepared on the substrate 80 which may comprise Si, glass orAl₂ O₃ -TiC without the overlying underlayer 82, especially in thoseapplications when a read-only sensor is to be formed. Alternatively, thespin valve structure can also be placed in between two soft magneticmaterials called shields wherein one shield can serve as a magnetic polefor the write-transducer. Another possibility is to place the spin valveread-transducer in between the two soft magnetic materials called poleswhich are part of write-transducer. In the latter two instances,nonmagnetic insulating gap layers such as Al₂ O₃ or SiO₂ may be providedbetween the spin valve structure and a pair of soft magnetic materialfilms.

With reference additionally now to FIGS. 4A-4C, various alternativeconstructions for a pinning layer are illustrated for placement upon thestructure previously illustrated in FIG. 3. In FIG. 4A, approximately50˜300 Å of an antiferromagnetic material such as FeMn, NiMn, or NiCoOis utilized as a pinning layer 86 overlying a thin buffer layer 84 suchas Cu or NiCr (having a thickness of about 20˜500 Å) to promote theproper microstructure and phase of the antiferromagnetic film,particularly where Mn is involved. The buffer layer 84 also functions toprevent interdiffusion between the underlayer 82 and theantiferromagnetic pinning layer 86. The pinning layer 86 underlies abottom ferromagnetic layer 88 of about 20˜200 Å of a transition metal orits alloys having its magmetization direction pinned in a perpendiculardirection to the ABS.

With reference to FIG. 4B, magnetic pinning may also be achieved byplacing a permanent magnetic pinning layer 92 underneath theferromagnetic layer to be pinned and overlying a buffer layer 90 whichmay range from approximately 0˜100 Å of Cr. Permanent magnets, such asCo or its alloys, have advantages in this pinning function over Mn-basedantiferromagnets (as previously described with respect to FIG. 4A) withrespect to material reliability problems such as corrosion resistance.Utilizing this alternative technique, the bottom ferromagnetic layer 94is exchange coupled with the permanent magnet. Therefore, themagnetization direction of the pinned bottom ferromagnetic layer 94 isparallel to that of permanent magnet pinning layer and both of thedirections are perpendicular to the magnetic storage media, or ABSsurface.

With reference to FIG. 4C, the bottom ferromagnetic layer 98 and thepermanent magnet pinning layer 92 are coupled magnetostatically byplacing a nonmagnetic separation layer 96 of about 10˜100 Å of Ta or Crin between them. Utilizing this technique, the magnetization directionof the pinned bottom ferromagnetic layer 98 is antiparallel to that ofpermanent magnet pinning layer 92, and both of the directions areperpendicular to the media, or ABS surface.

With respect to the alternative pinning techniques shown in FIGS. 4A-4C,the direction of the magnetization can be set in-situ under vacuumconditions or ex-situ, and any transition metal such as Ni, Fe, Co, orits alloys having a thickness of between substantially 20˜200 Å can beused for the ferromagnetic layers.

With reference additionally now to FIG. 5, the spin valve structure maybe defined on the substrate 80 and underlayer 82 (if utilized) includinga buffer layer 100 (comprising any of the alternative buffer layers 84,90 described with respect to Figs. 4A-4C) and a pinning layer 102(comprising any of the alternative pinning layers 86, 92 described withrespect to FIGS. 4A-4C). The bottom ferromagnetic layer 104 comprisesany of the bottom ferromagnetic layers 88, 94 or 98 above described withrespect to FIGS. 4A-4C and may be additionally separated from thepinning layer 102 by a separation layer 96 as previously described.

A spacer layer 106 is then deposited on the bottom ferromagnetic layer104 and may be provided as a 10˜50 Å thick Cu, Ag, Au, or Pd film ortheir alloys. A top ferromagnetic layer 108 is then deposited on thespacer layer 106 and may also comprise a 20˜200 Å thickness of atransition metal such as Ni, Fe, Co, or their alloys. The resultant filmstructure is patterned with photoresist 110 to define an active spinvalve device region as will be more fully described hereinafter.

With reference additionally to FIG. 6, it can be seen that those upperportions of the film structure comprising the top ferromagnetic layer108 and underlying spacer layer 106 not covered by the photoresist 110are etched away by an ion-milling process. Because of the so-called"shadowing effect" provided by the patterned photoresist 110, a taperedmesa-like structure for the active sensor layer comprising the topferromagnetic layer 108 can be formed. A desirable taper angle for themesa 112 is an acute angle of substantially 45° or less. In processing,the ion-milling rate is controlled so that the milling operation canstop precisely at the upper surface of the pinned bottom ferromagneticlayer 104. Alternatively, a slight undercut to the pinned bottomferromagnetic layer 104 can be purposefully formed.

With additional reference to FIG. 7, the subsequent deposition processesfor additional permanent magnet layers 118, 120 for domain stabilizationpurposes, and formation of the conductive current leads 122, 124 isshown. A pair of permanent magnet layers of about 0˜500 Å may bedeposited directly on top of the tapered side portions of the mesa 112or deposited after formation of a separation layers 114, 116(comprising, for example 0˜50 Å Cr or other suitable non-magnetic ordielectric material) with the presence of the photoresist 110 shown inFIG. 6. Following the permanent magnet layer 118, 120 deposition, thephotoresist is removed. A similar photolithographical process isfollowed to form the current leads 122, 124. When no permanent magnetlayers 118, 120 are needed for domain stabilization of the spin valvesensor, the current leads 122, 124 may be alternatively depositeddirectly on top of the tapered film structure of the mesa 112.

With additional reference to FIG. 8, the final processing steps areshown which include the deposition of a capping layer 126 and a top gaplayer 128. The capping layer 126, which may comprise substantially 0˜100Å of Ta is useful to protect the upper surface of the top ferromagneticlayer 108 in the area not overlain by the permanent magnet layers 118,120 as well as the permanent magnet layers 118, 120 themselves from theenvironment. The gap layer 128 may be formed of materials similar tothat of the underlayer 82.

What has been provided is a magnetoresistive device, or transducer, andmethod for manufacturing the same which includes a spin valve structurecomprising a pinned, bottom ferromagnetic layer and an active, topferromagnetic layer separated by a thin nonmagnetic metal spacer layer.The active ferromagnetic layer and underlying spacer layer are formedinto a mesa structure having tapered opposing sides to promote bettersurface planarization in a thin film fabrication process. A pair ofpermanent magnet layer portions may be deposited at the end portions ofthe spin valve mesa structure in a generally coplanar relationship topromote domain stabilization and may be separated therefrom by arelatively thin separation layer. The magnetic read track width of thedevice can be accurately and reproducibly determined byphotolithographically defining the spacing between the permanent magnetlayer portions overlying the active ferromagnetic layer.

While there have been described above the principles of the presentinvention in conjunction with specific manufacturing processes, it is tobe clearly understood that this description is made only by way ofexample and not as a limitation to the scope of the invention.

What is claimed is:
 1. A process for forming a magnetoresistive devicecomprising the steps of:providing a substrate; overlying a pinning layeron said substrate; further overlying a first ferromagnetic layer on saidpinning layer; yet additionally overlying a spacer layer on said firstferromagnetic layer; yet further overlying a second ferromagnetic layeron said spacer layer; and removing a selected portion of said secondferromagnetic layer and said spacer layer to form a mesa on said firstferromagnetic layer having tapered opposing first and second sidesthereof.
 2. The process of claim 1 further comprising the stepof:interposing an underlayer on said substrate between said substrateand said pinning layer.
 3. The magnetoresistive device of claim 2further comprising the step of:additionally interposing a buffer layerbetween said underlayer and said pinning layer.
 4. The process of claim1 further comprising the step of:still additionally overlying first andsecond separation layer portions on said first ferromagnetic layer andsaid tapered opposing first and second sides of said mesa, saidseparation layer additionally overlying a selected portion of an uppersurface of said second ferromagnetic layer adjacent said taperedopposing first and second sides.
 5. The process of claim 4 furthercomprising the step of:still further overlying said first and secondseparation layer portions with first and second permanent magnet layerportions respectively.
 6. The process of claim 5 further comprising thestep of:also overlying first and second conductor layer portions on aselected portion of said first and second permanent magnet layerportions respectively.
 7. The process of claim 6 further comprising thestep of:also further overlying a capping layer on said first and secondconductor layer portions, said first and second permanent magnet layerportions and said upper surface of said mesa between said first andsecond permanent magnet layer portions.
 8. The process of claim 1wherein said step of providing is carried out by means of a substrateselected from a group consisting of Si, A1₂ O₃ -TiC and glass.
 9. Theprocess of claim 2 wherein said step of interposing is carried out bymeans of a material selected from a group consisting of SiO₂ and A1₂ O₃.10. The process of claim 3 wherein said step of additionally interposingis carried out by means of a material selected from a group consistingof Cu and Nicr.
 11. The process of claim 1 wherein said step ofoverlying is carried out by means of an antiferromagnetic materialselected from a group consisting of FeMn, NiMn and NiCoO.
 12. Theprocess of claim 11 wherein said step of overlying is carried out bymeans of a permanent magnet material selected from a group consisting ofCo and its alloys.
 13. The process of claim 1 wherein said step offurther overlying is carried out by means of material selected from agroup consisting of transition metals and alloys.
 14. The process ofclaim 1 wherein said step of yet additionally overlying is carried outby means of material selected from a group consisting of Cu, Ag, Au, Pdand their alloys.
 15. The process of claim 1 wherein said step of yetfurther overlying is carried out by means of material selected from agroup consisting of transition metals and alloys.
 16. The process ofclaim 1 wherein said step of removing comprises the steps of:patterningphotoresist on said second ferromagnetic layer; and ion milling saidsecond ferromagnetic layer and said spacer layer to expose said firstferromagnetic layer.
 17. The process of claim 4 wherein said step ofstill additionally overlying is carried out by means of Cr.
 18. Theprocess of claim 5 wherein said step of still further overlying iscarried out by means of material selected from a group consisting of Coand its alloys.
 19. The process of claim 6 wherein said step of alsooverlying is carried out by means of material selected from a groupconsisting of Cu, Aft and Au.
 20. The process of claim 7 wherein saidstep of also further overlying is carried out by means of Ta.